How can I disable bpy.ops.convert objects sorting?
Run the operator once for all curves

I imagine you are getting poor performance because you are running the operator for each curve object, rather than once for all.  See Python performance with Blender operators

Suggest selecting all the curves in scene, and running the operator once, example test code.

import bpy

context = bpy.context

# select only curves
for o in context.scene.objects:
    o.select = o.type == 'CURVE'

curves = context.selected_objects
if curves:
    # just in case context object not a curve
    if not context.object.select:
        context.scene.objects.active = curves[0]
    # convert them all
    bpy.ops.object.convert(target='MESH')


As for the sorting, that's just what blender does in its background processing, and is I doubt responsible for slowing down your blender.

There is also the Object.to_mesh which can be used to convert the curve to mesh without using the operator. However in this case I think selecting all curves and converting with one operator call is sufficient.